1. Field of the Invention
The present invention relates to a slippage preventing apparatus for a vehicle.
2. Description of Related Art
Prevention of a large degree of slippage of the driven wheels on pavement permits an effective provision of an impellent force with the vehicle and is effective from a standpoint of safety in order to prevent the spinning of the vehicle. It is known that an increase in an amount of slippage of the driven wheel may be prevented by decreasing a torque to be applied to the driven wheels, which may cause slippage.
Japanese Patent Publication (laid-open) Nos. 16,948/1983 and 56,662/1985 disclose a slip control in which a torque to the driven wheels is decreased by applying a braking force to the driven wheels through the brake and by decreasing a torque output from the engine. More specifically, Japanese Patent Publication (laid-open) No. 16,948/1983 is directed to such a slip control as solely braking the driven wheels, on the one hand, when a degree of slippage is smaller, and as additionally decreasing the torque output from the engine, on the other hand, when a degree of slippage becomes larger. Japanese Patent Publication (laid-open) No. 56,662/1985 is directed to a slip control technology such that, when one of the left-hand and right-hand driven wheels slips to a larger degree than the other, only the one driven wheel is braked and, when the left-hand and right-hand driven wheels both slip to a greater degree, an output torque from the engine is reduced in addition to the braking against the driven wheels.
In order to implement the slip control against the driven wheel or wheels, it is necessary to detect a degree of slippage of the driven wheel or wheels. In the following description, although a degree of slippage will sometimes be referred to as a slip amount or value or ratios, it is to be understood that there is no substantial difference in meaning therebetween. Such a slip value can basically be determined as a ratio of a rotational speed of a driven wheel to a vehicle speed. However, it is found considerably difficult to directly detect a vehicle speed and, if possible, a detector sensor is rendered so extremely expensive that automobiles cannot be generally provided with such a sensor. Accordingly, given usual automobiles with either front or rear wheels being driven wheels and the other being undriven wheels, an attempt has been made that a vehicle speed is determined from a rotational speed of the undriven wheel which is not subject to the influence of a driving torque of the engine and a slip value of the driven wheel is given on the basis thereof. In this case, a slip value of the driven wheel is given as a ratio of a rotational speed of the driven wheel to a rotational speed of the corresponding undriven wheel.
As this slip value is computed on the basis of only two parameters of the rotational speeds of the driven and undriven wheels, the slip value cannot be accurately detected particularly at cornering, thus making a precise slip control unlikely to be executed. At cornering, a locus of the front wheel deviates from a locus of the rear wheel. Given the front wheels being driven, merely comparing the rotational speeds of the driven front wheels with those of the undriven rear wheels may lead to the false judgment that the driven front wheels slip apparently at cornering even if both the front and rear wheels do not slip whatsoever. In other words, an apparent slip value of the driven front wheel becomes larger than that of the undriven rear wheel. This may be applicable to an automobile of the rear wheel drive type although, in this case, an apparent slip value is smaller to the contrary.
This problem arises because a vehicle speed is determined as a rotational speed of the undriven wheel for reasons of convenience although the vehicle speed should be eventually determined at a reference position of the vehicle body and because the driven wheels are apart from the reference position thereof. This allows a cornering radius of the reference position thereof to deviate from that of the driven wheel at cornering. Thus this problem may likewise occur even if an absolute vehicle speed against pavement could be detected using a vehicle speed sensor or at an automobile of four wheel drive type.
This phenomenon implies that a reduced amount of a torque to be applied to the driven wheels does not function as an optimum factor when the torque is reduced upon the slip control.